1. Field of the Invention
The present invention relates to a method for producing xylylenediamine by hydrogenating phthalonitrile obtained through ammoxidation of xylene.
Xylylenediamine is useful as a raw material of, for example, polyamide resins or epoxy curing agents, and as an intermediate material for producing isocyanates.
2. Background
In a well-known method for producing phthalonitrile, xylene is reacted through ammoxidation with ammonia and molecular oxygen in the presence of a catalyst. For example, Japanese Patent Application Laid-Open (kokai) No. Heisei 11(1999)-209332 discloses a method for producing phthalonitrile through ammoxidation by use of a catalyst containing a V-Cr-B-Mo oxide. The thus-produced phthalonitrile is hydrogenated in the presence of ammonia, to thereby produce xylylenediamine.
A gas produced during production of phthalonitrile through ammoxidation of xylene contains phthalonitrile (i.e., a target product), ammonia, carbon dioxide gas, carbon monoxide, hydrogen cyanide, aromatic amide, aromatic carboxylic acid, air, and steam. Therefore, before the phthalonitrile is subjected to hydrogenation, it must be separated from the produced gas through trapping.
In a proposed method for trapping and separating phthalonitrile from a gas produced through ammoxidation, the produced gas is introduced into a cooling apparatus having a large surface area; phthalonitrile is deposited and solidified on a cooling surface; and then phthalonitrile is collected after being melted. However, phthalonitrile is prone to undergo undesirable change (e.g., polymerization) at high temperature, and deteriorates when melted and removed, resulting in lowering of the purity of a product.
In a proposed method similar to the aforementioned method, the produced gas is introduced into a cooling apparatus; phthalonitrile is deposited and solidified on a cooling surface; a solvent is added to the solidified phthalonitrile; and the resultant mixture is fed to a hydrogenation reactor (Kagaku Kogaku, Vol. 32, No. 7, pp. 658-660 (1968)). However, in this method, phthalonitrile is prone to undergo change (e.g., polymerization) on the cooling surface of the cooling apparatus, to thereby generate a polymer which is insoluble in the added solvent, leading to maloperation of the apparatus due to accumulation of the polymer.
In another proposed trapping method, a gas produced through ammoxidation and containing phthalonitrile is brought into direct contact with water, phthalonitrile crystals are trapped while being suspended in the water, and phthalonitrile is separated from the suspension through solid-liquid separation (Process Handbook, edited by The Japan Petroleum Institute (1978)). Through this method, phthalonitrile is satisfactorily trapped. However, since the bulk specific gravity of phthalonitrile in the suspension is low, the slurry has a large volume. Therefore, separating solid and liquid from the aqueous slurry solution through filtration requires a very large filtration apparatus. In addition, the water content of crystals separated from the slurry solution is high, and thus a large amount of heat is necessary for drying the crystals.
Since phthalonitrile is relatively easily reacted with water at high temperature, to thereby form an amide of high boiling point, long-term heating of phthalonitrile in the presence of water causes lowering of the purity of phthalonitrile. When water is used as a trapping solvent, hydrogen cyanide, which is a by-product, is brought into contact with water at high temperature. Through the thermal process, hydrogen cyanide is easily transformed into, for example, formamide, ammonium formate, or a polymer. When such a substance is contained in wastewater, the substance causes an increase in TOD load and coloring of wastewater.
In still another proposed method for trapping and separating phthalonitrile from a gas produced through ammoxidation, the gas is brought into contact with an organic solvent (Process Handbook, edited by The Japan Petroleum Institute (1976)). In this method, after a solution in which phthalonitrile is trapped is subjected to distillation to thereby recover a solvent, rectification is carried out so as to purify phthalonitrile. Therefore, a large amount of energy is required for purification, and a large amount of phthalonitrile is lost.
In the subsequent step, phthalonitrile produced through ammoxidation is dissolved in ammonia or an organic solvent, and the resultant solution is subjected to hydrogenation.
When phthalonitrile is obtained in the form of solid or melt, a dissolution bath or a mixing bath is required for dissolving the phthalonitrile uniformly in a solvent prior to hydrogenation.
As described above, the conventional techniquesxe2x80x94in which phthalonitrile is separated, through trapping, from a gas produced through ammoxidation, and the phthalonitrile is subjected to hydrogenationxe2x80x94have drawbacks; for example, the purity of phthalonitrile is lowered due to generation of by-products during separation of phthalonitrile from a gas produced through ammoxidation, waste or wastewater increases, and a large amount of energy is required.
In recent years, demand has arisen for polyamide resins or similar materials which are not easily colored, and therefore, xylylenediamine of higher purity is desired.
Thus, an object of the present invention is to provide a method for producing xylylenediamine by hydrogenating phthalonitrile synthesized through ammoxidation of xylene, comprising recovering phthalonitrile, readily and at high yield, from a gas produced through ammoxidation, to thereby efficiently produce xylylenediamine of high purity.
In view of the foregoing, the present inventors have performed extensive studies, and have found that, by bringing a gas produced through ammoxidation into direct contact with an organic solvent (A), phthalonitrile can be trapped in the organic solvent (A), and through hydrogenation including adding liquid ammonia to the resultant mixture without separation of phthalonitrile trapped in the organic solvent (A), the phthalonitrile can be readily recovered from the produced gas and at high yield without need for new equipment, and xylylenediamine can be efficiently produced through hydrogenation; and that, by subjecting the produced xylylenediamine to extraction by use of an organic solvent (B) and water, xylylenediamine of high purity can be obtained. The present invention has been accomplished on the basis of these findings.
Accordingly, the present invention provides a method for producing xylylenediamine by hydrogenating phthalonitrile synthesized through ammoxidation of xylene, which method comprises:
(1) an ammoxidation step for producing phthalonitrile by causing xylene to react in a vapor-solid catalytic manner with ammonia and an oxygen-containing gas, to thereby cause ammoxidation;
(2) a trapping step for trapping phthalonitrile in an organic solvent (A) by bringing a gas produced through ammoxidation into direct contact with the organic solvent (A); and
(3) a hydrogenation step for carrying out hydrogenation including adding liquid ammonia to phthalonitrile without separating the phthalonitrile trapped in the organic solvent (A).
There is also provided a method for producing xylylenediamine according to the aforementioned method, which method further comprises:
(4) a separation step for separating ammonia and the organic solvent (A) or ammonia from a hydrogenation product, to thereby obtain crude xylylenediamine;
(5) an extraction step for adding to the crude xylylenediamine water, or water and an organic solvent (B), to thereby separate the resultant mixture into an organic solvent phase and an aqueous phase; and
(6) a recovery step for recovering xylylenediamine of high purity from the aqueous phase which has been separated through extraction.